If you want to be on the latest Samsung official Jelly Bean ROM with the newer features like Multi-window and some more, check out the WanamLite Jelly Bean ROM for your international Galaxy S3 GT-i9300. XDA user Wanam is probably one of the best ROM developers and this ROM represents some of his best work. You will get full stock features plus many enhancements like 15 toggles, tweaked kernels for better performance/battery life, and a ton more.

Check it out and let me know what you think over other Jelly Bean ROMs out right now, I think you will like it.